1. Field of the Invention
The present invention provides a semiconductor plasma processing method and system using novel combinations of gases including C.sub.5 F.sub.8. In an exemplary preferred form of the invention, C.sub.5 F.sub.8 is utilized in combination with CO and/or O.sub.2 in the presence of a carrier gas, e.g., Argon.
2. Discussion of the Background
Integrated circuits and other electrical devices are today manufactured utilizing plural plasma processing steps, in which the plasma interacts with a substrate (e.g., a semiconductor wafer) to (1) deposit material onto the substrate in layers, or (2) etch the various layers formed on the substrate. Deposition and etching are not always mutually exclusive since, e.g., during an etching operation, materials can also be deposited.
When fabricating semiconductor devices, numerous regions having different electrical properties (e.g., conductive regions, non-conductive regions, etc.) are formed in layers on and upon the semiconductor substrate. The conductive regions include a semiconductor substrate, a source or drain region, the gate material of a gate electrode, and a conductive material. Non-limiting examples of suitable conductive regions include a metal such as aluminum, polysilicon (which may be conventionally doped with n-dopants such a phosphorous, arsenic, antimony, sulfur, etc., or with p-dopants such as boron), titanium, tungsten, copper, and conductive alloys thereof such as aluminum-copper alloy and titanium-tungsten alloy.
The conductive regions and layers of the device are isolated from one another by a dielectric, for example, silicon dioxide. The silicon dioxide may be (1) grown, (2) deposited by physical deposition (e.g., sputtering), or (3) deposited by chemical deposition. Additionally, the silicon dioxide may be undoped or doped, for example, with boron, phosphorus, or both, to form borosilicate glass (BSG), phosphosilicate glass (PSG), and borophosphosilicate glass (BPSG), respectively. The method used to form and dope a silicon dioxide layer will depend upon various device and processing considerations. Herein, all such silicon dioxide layers are referred to generally as "oxide" layers.
At several stages during fabrication, it is necessary to make openings in a dielectric layer to contact underlying regions or layers. Generally, an opening through a dielectric layer between polysilicon and the first metal layer is called a "contact opening," while an opening in other oxide layers such as an opening through an intermetal dielectric layer (ILD) is referred to as a "via." As used herein, an "opening" will be understood to refer to any type of opening through any type of oxide layer, regardless of the stage of processing, layer exposed, or function of the opening.
The positions and sizes of the openings are defined by photolithographic masks. Typically, a photosensitive film (or resist) is deposited on the surface of a dielectric layer, and the photolithographic mask blocks a portion of the light which would otherwise expose a corresponding portion of the film when the film is exposed to a light of a known intensity and frequency. Suitable photoresist materials are those conventionally known to those of ordinary skill in the art and may comprise either positive or negative photoresist materials. Either or both positive and/or negative resist layers may be used. The photoresist may be applied by conventional methods known to those of ordinary skill in the art.
Negative resist materials may contain chemically inert polymer components such as rubber and/or photoreactive agents that react with light to form cross-links, e.g. with the rubber. When placed in an organic developer solvent, the unexposed and unpolymerized resist dissolves, leaving a polymeric pattern in the exposed regions. The preparation of suitable negative resist materials is within the level of skill of one of ordinary skill in the art without undue experimentation. Specific non-limiting examples of suitable negative resist systems include cresol epoxy novolac-based negative resists as well as negative resists containing the photoreactive polymers described in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, Vol. 17, entitled "Photoreactive Polymers", pages 680-708, the relevant portions of which are hereby incorporated by reference.
Positive resists have photoreactive components which are destroyed in the regions exposed to light. Typically the resist is removed in an aqueous alkaline solution, where the exposed region dissolves away. The preparation of suitable positive resist materials is within the level of skill of one of ordinary skill in the art without undue experimentation. Specific non-limiting examples of suitable positive resist systems include Shipley XP9402, JSR KRK-K2G and JSR KRF-L7 positive resists as well as positive resists containing the photoreactive polymers described in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, Vol. 17, entitled "Photoreactive Polymers", pages 680-708, the relevant portions of which are hereby incorporated by reference.
Exemplary resist materials are also described by Bayer et al., IBM Tech. Discl. Bull. (USA) Vol. 22, No. 5, (Oct. 1979), pp. 1855; Tabei, U.S. Pat. No. 4,613,404; Taylor et al., J. Vac. Sci., Technol. Bull. Vol. 13, No. 6, (1995), pp. 3078-3081; Argritis et al., J. Vac. Sci., Technol. Bull., Vol. 13, No. 6, (1995), pp. 3030-3034; Itani et al., J. Vac. Sci, Technol. Bull. Vol. 13, No. 6, (1995), pp. 3026-3029; Ohfuli et al., J. Vac. Sci, Technol. Bull. Vol. 13, No. 6, (1995), pp. 3022-3025; Trichkov et al., J. Vac. Sci., Technol. Bull. Vol. 13, No. 6, (1995), pp. 2986-2993; Capodieci et al., J. Vac. Sci, Technol. Bull. Vol. 13, No. 6, (1995), pp. 2963-2967; Zuniga et al., J. Vac. Sci, Technol. Bull. Vol. 13, No. 6, (1995), pp. 2957-2962; Xiao et al., J. Vac. Sci, Technol. Bull. Vol. 13, No. 6, (1995), pp. 2897-2903; Tan et al., J. Vac. Sci, Technol. Bull. Vol. 13, No. 6, (1995), pp. 2539-2544; and Mayone et al., J. Vac. Sci, Technol. Vol. 12, No. 6, pp. 1382-1382. The relevant portions of the above-identified references which describe the preparation of resist materials are hereby incorporated by reference.
In recent years, the degrees of integration of semiconductor devices have been greatly improved, and accordingly, the size reduction of various elements formed on semiconductor substrates has become one of the essential technical requirements. In order to meet such a requirement, it is necessary to reduce the gap between respective gates (electrodes) formed above a semiconductor substrate, and when a contact hole is formed between such gates, it is also necessary to reduce the size of the contact hole. As the gap between gates has decreased, it has become difficult to form a microscopic contact hole at an accurate position due to limitations on the stepper alignment, etc. In recent years, therefore, a self-aligned contact method has been used where a protective film (base) (such as a silicon nitride(SiN.sub.x) film) is formed on the surface of each gate to prevent the etching of the gates during the formation of a contact hole and wherein a contact hole is formed in the microscopic space between adjacent gates in a self-aligning manner.
Prior to etching, a photoresist (or film) is applied, exposed, and developed. Development of the film removes a portion of the film, thereby forming a pattern in which portions of the oxide are exposed. The exposed portions of the oxide may then be subject to selective etching to form a contact. Plasma-based etching processes such as reactive ion etching (RIE) are very common, however etching can also be performed by other methods, such as using a high density chamber or a DRM chamber. In the reverse process, deposition can be performed by plasma enhanced chemical vapor deposition. Typically, the plasma is generated by coupling radio frequency (RF) electro-magnetic energy to the plasma. The RF energy is supplied by an RF generator coupled to a power supply. Since the plasma has a variable impedance, a matching network is employed to match the impedance of the power supply with that of the plasma. The matching network may include one or more capacitors and one or more inductors to achieve the match and thereby tune the RF power. Typically, the tuning may be done automatically by an automatic matching network (AMN). When tuned, most of the power output of the RF generator is coupled to the plasma. The power to the plasma is often referred to as forward power.
Etch characteristics are generally believed to be affected by polymer residues which deposit during the etch. For this reason, the fluorine to carbon ratio (F/C) in the plasma is considered an important factor in the etch. In general, a plasma with a high F/C ratio will have a faster etch rate than a plasma with a low F/C ratio. At very low F/C ratios (i.e., high carbon content), polymer deposition may occur and etching may be reduced. The etch rate as a function of the F/C ratio is typically different for different materials. This difference is used to create a selective etch, by attempting to use a gas mixture which puts the F/C ratio in the plasma at a value that leads to etching at a reasonable rate for one material, and that leads to little or no etching or polymer deposition for another. For a more thorough discussion of oxide etching, see S. Wolf and R. N. Tauber, Silicon Processing for the VLSI ERA, Volume 1, pp 539-585 (1986), the contents of which are incorporated herein by reference. The introduction of oxygen into an etching process has been reported to allow for control of the anisotropy, by varying the fraction of O.sub.2 in the feed. For example, see Burton et al., J. Electrochem. Soc.: Solid-State Science and Technology, v 129, no 7, 1599 (1982), the contents of which are incorporated herein by reference.
A number of gases have been used in known systems to etch oxide layers, including CF.sub.3, CF.sub.4, and CH.sub.2 F.sub.2. By selecting the appropriate gas for use in the etching process, some layers are selectively etched while leaving other layers (etch stops) relatively unharmed. CH.sub.2 F.sub.2 has also been used and tends to provide a passivation layer on horizontal surfaces. A mixed gas obtained by adding CO to C.sub.4 F.sub.8 is known to be used during etching to form contact holes, especially during an etching process which forms a contact hole between gates and through an insulating film such as an SiO.sub.2 film.
C.sub.4 F.sub.8, however, is not easily decomposed in the atmosphere. Consequently, any C.sub.4 F.sub.8 that is not dissociated during processing and which is subsequently released into the atmosphere contributes to greenhouse effects and thus accelerates global warming. In other words, according to "PFC Problems in Semiconductor Mass Production Plants: Current States and Countermeasures," Climate Change 1995, the atmospheric life of C.sub.4 F.sub.8 is 3,200 years, whereas the atmospheric life of C.sub.5 F.sub.8 is 0.3 years.
Previous unsuccessful attempts have been made to use C.sub.5 F.sub.8 in a plasma processing systems. Such attempts were unsuccessful since they failed to provide the required selectivity. It is believed that a major impediment to the use of C.sub.5 F.sub.8 was the low purity of previously available C.sub.5 F.sub.8 gases. Generally, available C.sub.5 F.sub.8 gases were only 95-97% pure. As a result, contamination created an unacceptable, non-uniform product because of non-uniform etch and deposition rates.
Accordingly, C.sub.5 F.sub.8 was not previously accepted as an etchant for uniformly processing semiconductor substrates. Furthermore, the high selectivity of C.sub.5 F.sub.8 for oxide versus silicon nitrides was unproven. C.sub.5 F.sub.8, in the form of octofluro-cyclopentene is now available in high purity from Nippon Zeon Co., Ltd. of 2-6-1 Marunouchi, Chiyoda-ku, Tokyo 100 Japan.